Self defense is something you need to learn from a good teacher. Reading from books is fine if you just want to fine tune things or pick up some generic tips, but only a master versed in the art of self defense can teach you the essential body positions and movements.
A great reason to take up self defense training is to reduce stress. A round with the punching bag after an exhausting day at work can be a great antidote to stress and tension. A lot of people find that sparring with a partner can be therapeutic.
If you want to get into shape, self defense training is a great option. Any martial art requires the body to be balanced, strong, and agile. You can't really achieve this without regular training. As part of your training, you will likely indulge in strength training, extensive cardio, exercises to improve your agility, speed, and balance - elements that contribute to a fitter, healthier you.
Finally, one self defense tip you should always follow, regardless of your training: always carry an easily accessible and concealable weapon with you, such as a stun gun, a pepper spray, or even a pen, especially if you are a woman. These can make getting out of a tricky situation a whole lot easier.